Electrical signals can be encoded onto fiber-optic transmissions by electro-optic modulators. These modulators include electro-optic materials having highly polarizable electrons. When these materials are subject to an electric field, their polarization changes dramatically resulting in an increase in the index of refraction of the material and an accompanying decrease in the velocity of light traveling through the material. This electric field-dependent index of refraction can be used to encode electric signals onto optical signals. Uses include, for example, switching optical signals and steering light beams.
A variety of electro-optic materials have been utilized for use in electro-optic devices. Among these materials are inorganic materials such as lithium niobate, semiconductor materials such as gallium arsenide, organic crystalline materials, and electrically poled polymer films that include organic chromophores. A review of nonlinear optical materials is provided in L. Dalton, “Nonlinear Optical Materials,” Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Vol. 17 John Wiley & Sons, New York, pp. 288-302 (1995).
In contrast to inorganic materials in which polar optical lattice vibrations diminish effectiveness, the optical properties of organic nonlinear optical materials depend primarily on the hyperpolarizability of their electrons without a significant adverse contribution from the lattice polarizability. Thus, organic nonlinear optical materials offer advantages for ultrafast electro-optic modulation and switching.
Lithium niobate, a common material currently utilized in electro-optic devices, has an electro-optic coefficient of about 35 pm/V resulting in a typical drive voltage of about 5 volts. Drive voltage (Vπ) refers to the voltage that produces a π phase shift of light. Lithium niobate has a high dielectric constant (∈=28), which results in a mismatch of electrical and optical waves propagating in the material. The mismatch necessitates a short interaction length, which makes drive voltage reduction through increasing device length unfeasible, thereby limiting the device's bandwidth. Recent lithium niobate modulators have been demonstrated to operate at a bandwidth of over 70 GHz.
Electro-optic poled polymers have also been utilized as modulating materials. Their advantages include their applicability to thin-film waveguiding structures, which are relatively easily fabricated and compatible with existing microelectronic processing. These polymers incorporate organic nonlinear optically active molecules to effect modulation. Because organic materials have low dielectric constants and satisfy the condition that n2=∈, where n is the index of refraction and ∈ is the dielectric constant, organic electro-optic will have wide bandwidths. The dielectric constant of these materials (∈=2.5-4) relatively closely matches the propagating electrical and optical waves, which provides for a drive voltage in the range of about 1-2 volts and a bandwidth greater than 100 GHz.
Advantages of organic nonlinear optical materials include a bandwidth in excess of 100 GHz/cm device and ease of integration with semiconductor devices. See L. Dalton et al., “Synthesis and Processing of Improved Organic Second-Order Nonlinear Optical Materials for Applications in Photonics,” Chemistry of Materials, Vol. 7, No. 6, pp. 1060-1081 (1995). In contrast to inorganic materials, these organic materials can be systematically modified to improve electro-optic activity by the design and development of new organic materials and by the development of improved processing methods. See L. Dalton et al., “The Role of London Forces in Defining Noncentrosymmetric Order of High Dipole Moment-High Hyperpolarizability Chromophores in Electrically Poled Polymeric Films,” Proceedings of the National Academy of Sciences USA, Vol. 94, pp. 4842-4847 (1997).
For an organic nonlinear optical material to be suitable for electro-optic applications, the material should have a large molecular optical nonlinearity, referred to as hyperpolarizability (β), and a large dipole moment (μ). A commonly figure of merit used to compare materials is the value μβ. See Dalton et al. (1997). Organic materials having μβ values greater than about 15,000×10−48 esu that also satisfy the desired thermal and chemical stability and low optical loss at operating wavelengths have only recently been prepared. See Dalton et al., “New Class of High Hyperpolarizability Organic Chromophores and Process for Synthesizing the Same,” WO 00/09613. However, materials characterized as having such large μβ values suffer from large intermolecular electrostatic interactions that lead to intermolecular aggregation resulting in light scattering and unacceptably high values of optical loss. See Dalton et al. (1997). A chromophore's optical nonlinearity (μβ) can be measured as described in Dalton et al., “Importance of Intermolecular Interactions in the Nonlinear Optical Properties of Poled Polymers,” Applied Physics Letters, Vol. 76, No. 11, pp. 1368-1370 (2000). A chromophore's electro-optic coefficient (r33) can be measured in a polymer matrix using attenuated total reflection (ATR) technique at telecommunication wavelengths of 1.3 or 1.55 μm. A representative method for measuring the electro-optic coefficient is described in Dalton et al., “Importance of Intermolecular Interactions in the Nonlinear Optical Properties of Poled Polymers,” Applied Physics Letters, Vol. 76, No. 11, pp. 1368-1370 (2000).
Many molecules can be prepared having high hyperpolarizability values, however their utility in electro-optic devices is often diminished by the inability to incorporate these molecules into a host material with sufficient noncentrosymmetric molecular alignment to provide a device with acceptable electro-optic activity. Molecules with high hyperpolarizability typically exhibit strong dipole-dipole interactions in solution or other host material that makes it difficult, to achieve a high degree of noncentrosymmetric order without minimizing undesirable spatially anisotropic intermolecular electrostatic interactions.
Chromophore performance is dependent on chromophore shape. See Dalton et al., “Low (Sub-1-Volt) Halfwave Voltage Polymeric Electro-optic Modulators Achieved by Controlling Chromophore Shape,” Science, Vol. 288, pp. 119-122 (2000).
Chemical, thermal, and photochemical stabilities are imparted to the chromophores through their chemical structure and substituent choice. For example, in certain embodiments, the chromophore's active hydrogens are substituted with groups (e.g., alkyl, fluorine) to impart increased stability to the chromophore.
Thus, the effectiveness of organic nonlinear optical materials having high hyperpolarizability and large dipole moments can be limited by the tendency of these materials to aggregate when processed into electro-optic devices. The result is a loss of optical nonlinearity. Accordingly, improved nonlinear optically active materials having large hyperpolarizabilities and large dipole moments and that, when employed in electro-optic devices, exhibit large electro-optic coefficients may be advantageous for many applications.
For the fabrication of practical electro-optical (E-O) devices, critical material requirements, such as large E-O coefficients, high stability (thermal, chemical, photochemical, and mechanical), and low optical loss, need to be simultaneously optimized. In the past decade, a large number of highly active nonlinear optical (NLO) chromophores have been synthesized, and some of these exhibit very large macroscopic optical nonlinearities in high electric field poled guest/host polymers. To maintain a stable dipole alignment, it is a common practice to utilize either high glass transition temperature (Tg) polymers with NLO chromophores as side chains or crosslinkable polymers with NLO chromophores that could be locked in the polymer network. However, it is difficult to achieve both large macroscopic nonlinearities and good dipole alignment stability in the same system. This is due to strong intermolecular electrostatic interactions among high dipole moment chromophores and high-temperature aromatic-containing polymers, such as polyimides and polyquinolines that tend to form aggregates. The large void-containing dendritic structures may provide an attractive solution to this critical issue because the dendrons can effectively decrease the interactions among chromophores due to the steric effect. Furthermore, these materials are monodisperse, well-defined, and easily purifiable compared to polymers that are made by the conventional synthetic approaches.